Desirable characteristics of a memory device include high storage density, low power consumption (during all modes of operation including reading, writing, and erasure), non-volatility (i.e., persistence of data without applied power), high long-term data integrity, rapid writing and erasure of data, and rapid addressing and reading of stored data. A variety of existing memory devices exhibit these characteristics in various respective measures. Devices are typically chosen for a particular application based on the requirements of the application. For example, where long-term data storage is required in a portable device (i.e., a device supplied by battery power), a non-volatile memory may be chosen.
Most non-volatile memories are relatively slow, as compared with volatile memories. Consequently, when rapid data storage and retrieval is required volatile memories may be applied. In some systems requiring both long-term data storage and rapid data transfer in a portable device, combinations of volatile and non-volatile memories are necessary. Such combinations tend to increase price, size, and power consumption of the target system.
Recently, variable resistance memory devices have been investigated as a way of providing a semi-volatile or non-volatile memory device. A non-volatile memory device requires no refreshing operations to maintain status, while a semi-volatile memory device requires refreshing, but at intervals that are much longer than the refresh intervals typically used for Dynamic Random Access Memory (DRAM) devices. For example, a semi-volatile memory device may need to have its memory elements refreshed every few minutes, hours or even days. Chalcogenide materials having the formula GexSe100-x may be formed into semi-volatile resistance variable memory elements.
Memory elements based on GexSe100-x backbone material are disclosed in U.S. application Ser. No. 09/941,544, filed on Aug. 30, 2001 and Ser. No. 10/225,190, filed on Aug. 22, 2002, assigned to Micron Technology, Inc., the disclosures of which are incorporated herein by reference. Such memory elements are also known as programmable conductive random access memory (PCRAM) elements.
Generally, a programmable conductive memory element includes an initially insulating dielectric material formed of a chalcogenide glass, e.g., GexSe100-x, disposed between two electrodes. A conductive material, such as silver, is incorporated into the dielectric material. The resistance of the dielectric material can be changed between high resistance and low resistance states. The memory is normally initially in a high resistance state. A write operation to a low resistance state is performed by applying an electrical potential across the two electrodes.
When set in a low resistance state, the state of the memory element will remain intact for minutes or longer after the electrical potentials is removed. The elements can be returned to their high resistance state by applying a different electrical potential between the electrodes than the potential used to write the element to the low resistance state. Again, the highly resistive state is maintained once the electrical potential is removed. Thus, the PCRAM element can function, for example, as a resistance variable memory element having two resistance states, which can be used to define two logic states.
As with any memory device, the areal storage density of the device using programmable conductive memory elements affects device cost, system size, power consumption, operating speed, and other factors. Accordingly it is desirable to produce a memory device employing such memory elements having a high areal data storage density.